Synonyms and related words :
bugging, chasing, cloak-and-dagger work, counterespionage, counterintelligence, dogging, electronic surveillance, espial, espionage, following, heeling, hounding, intelligence, intelligence work, military intelligence, observation, pursual, pursuance, pursuit, secret police, secret service, sequel, sequence, series, shadowing, spying, stakeout, surveillance, trailing, wiretap, wiretapping